1. Field of the Invention
This invention relates to a digital/analog (D/A) converter of differential current source type having a preset current source (which is the same as a constant current source) and selectively deriving the output of the current source by means of a differential switch.
2. Description of the Related Art
One unit circuit of an ordinary D/A converter (which is hereinafter referred to as a DAC) constituted by MOS transistors is shown in FIG. 1 as an example of a conventional D/A converter of differential current source type. A multi-bit input DAC comprises a plurality of circuits of the type shown in FIG. 1, whose outputs Q are connected together. In FIG. 1, 11 denotes a preset current source (constant current source), and 12.sub.3 and 13.sub.3 denote N-channel MOS transistors constituting differential switches, and the differential switches 12.sub.3 and 13.sub.3 are respectively provided on the dummy side and DAC side (DAC output deriving side). The drain of the transistor 12.sub.3 is connected to a power source VDD, and current outputs Q and Q indicate that a current may flow in one of the DAC output deriving path and the dummy path and no current is permitted to flow in the other current path. Control inputs .phi. and .phi. which are in the inverted relationship (complementary relationship) are digital inputs.
The multi-bit input DAC comprises, for example, 16 circuits of the type shown in FIG. 1. This is a 4-bit input DAC. Unlike a ladder resistor voltage dividing system, in this circuit system, a preset current is created according to a reference voltage and current outputs Q and Q are derived by turning on one of the differential switches 12.sub.3 and 13.sub.3 on the DAC side and dummy side in accordance with a decoded input code, thereby obtaining an output.
The above differential current system has an advantage that one of the current outputs Q and Q always flows along one of the current paths. Hence, the constant current will not vary and high speed operation can be attained, provided the outputs Q and Q have the same output impedance. However, with the actual device, since the output impedances of portions for the current outputs Q and Q are different from each other, a voltage (at a node B) of the preset current source 11 and a current (a Q output on the DAC side) transiently vary as shown in FIG. 2 when one of the current paths for Q and Q is changed over to the other current path, and a certain period of time will pass before the voltage at the node B becomes stable, thereby lowering the D/A conversion speed (settling time) and preventing the high speed operation.
In order to solve the above problem, the output terminals are provided on the Q and Q sides to make the impedances equal to each other in the prior art, but in this case, the number of terminals is increased, the number of parts used is increased and the cost thereof increases. Further, a means having a previously set impedance in a chip is provided on the Q side, but it is almost impossible to actually detect the correct values of an inductance (L). resistor (R) and capacitor (C) given by an externally attached impedance on the Q side and stably mount the impedance.
As described above, in the system in which the constant current is selected and output by means of the differential switch, it is ideal that the current values of Q (DAC side) and Q (dummy side) are equal to each other, but the current at the node B will vary when the current path is changed from the Q side current path to the Q side current path since the output impedances on the current paths are different from each other.